In fractured reservoirs, the fractures are good infiltration flow channels through which the reservoirs communicate with the well bore. However, when a fractured reservoir is encountered during drilling, the drilling fluid will intrude into the reservoir along the fractures under pressure difference, resulting in reservoir damages and decreased oil and gas output. Up to now, domestic and foreign researchers have made long-time researches on drilling fluid techniques for fractured reservoir protection, and have developed fractured reservoir protective drilling fluid systems. Existing fractured reservoir protective drilling fluids are mainly water-based drilling fluids, in which usually several materials work with each other to attain a reservoir protection effect. Those protective materials usually contain rigid granule (e.g., calcium carbonate), fiber and/or bitumen materials, etc., and attain an effect of plugging fractures and preventing intrusion of the drilling fluid by means of compounding of these constituents. The drilling fluid based on above-mentioned protective materials protects the reservoir mainly by means of a shielded temporary plugging mechanism.
Drilling fluids based on a shielded temporary plugging technique has the following drawbacks in protecting the fractured reservoir: Firstly, to attain a good shielded temporary plugging effect, the widths and distributional characteristics of the reservoir fractures must be ascertained; however, owing to the heterogeneous characteristic of the reservoir, it is difficult to seize the characteristics of the reservoir fractures accurately; consequently, the temporary plugging effect is often unsatisfactory; secondly, since the protective materials have to enter into the fractures to generate a plugging effect, the plugged zone blocks the oil and gas from flowing into the well bore while it blocks the drilling fluid from entering into the reservoir; therefore, plug removal has to be carried out after drilling. Hence, there is an urgent need for developing an innovative water-based drilling fluid, which is applicable to fractured reservoirs and has strong plugging capability, and can flow back easily, to overcome the drawbacks of the existing fractured reservoir protective drilling fluids.